In fluidized-bed boilers based on the circulating-powder technique, the mass ratio of circulating powder to flue gases is typically 20:1 to 50:1. An abundance of powder equalizes the temperature profile of the furnace in a circulating-powder boiler quite efficiently even though the combustion takes place mainly in the lower part of the furnace and the cooling in the upper parts. The difference between the maximal and minimal temperatures in the circulation circuit is, at the maximum, 100 K.
The capacity of cooling of the furnace of a circulating-powder boiler is typically 30% to 50% of the total capacity of the boiler. As a rule, the cooling of the furnace has been accomplished by means of membrane heat-exchanger faces placed on the walls of the furnace and protected by a thin protective masonwork. The shield is needed because of erosion caused by the powder and because of corrosion caused by the reducing conditions. Tube packages can be placed in the upper part of the furnace, where they do not have to be protected, because in the upper part the conditions are oxidizing and the risk of corrosion is no longer as high as in the combustion zone.
Lowering of the capacity of cooling of the furnace of a circulating-powder boiler is problematic in fluidized-bed combustion. Lowering of the temperature in the furnace can hardly be used for regulation, because then the conditions of combustion would become unfavorable.
Prior-art solutions for regulation of the capacity of cooling of the furnace include the following modes of cooling:
The regulation of the capacity of cooling of the furnace takes place so that the quantity of circulating powder is affected by means of the distribution of air for the furnace. The quantity of circulating powder affects the heat-transfer coefficient. If the furnace is not cooled, the temperature will rise up to 1500.degree. C. and the ashes will melt. In such a case, the fluidization of the circulating material in the reactor is disturbed. If the fluidization is disturbed, the combustion in the reactor is also disturbed.
For regulation of the capacity of cooling of the furnace, the method has also been used in which the hot circulating material that was separated in the powder separator after the furnace is recirculated directly into the combustion chamber. The circulating material has been cooled by means of separate heat-exchanger faces before returning into the combustion chamber. The heat-exchanger faces are placed in a separate fluidized bed, into which all or part of the hot circulating material is passed and from which the cooled circulating material is returned into the combustion chamber. The fluidization air of the separate fluidized bed is passed to the circulating-powder boiler as secondary air.
In the prior-art solutions, the dimensioning of the furnace cooling and the operation of the boiler with the use of fuels of different qualities have proved quite problematic even for the most experienced boiler manufacturers.
Along with the power level, the conditions of combustion in circulating-powder boilers have changed so extensively that optimal conditions for the removal of sulphur and nitrogen cannot be maintained within the entire capacity range.
The cooling of the circulating material by means of heat-exchanger faces is problematic because of particle erosion, corrosion, and increased costs.
Moreover, the sealing up of the power ranges of fluidized-bed boilers has proved difficult, because, owing to the internal circulation of material inside the furnace, the density of the circulating material on the furnace walls cannot be predicted precisely. This is why dimensioning of the heat-exchanger faces has not been successful.
The use of combustion air for regulation of the mount of circulating material and for regulation of the heat transfer has deteriorated the conditions of combustion in the lower part of the reactor and lowered the efficiency of the sulphur removal and of the combustion.